1. Field of the Invention
The invention relates to an arrangement for angular measurement.
2. Description of the Related Art
Known arrangements for angular measurements, also referred to as angle measuring systems, use, inter alia, the magnetoresistive effect, i.e. a change of the electric resistance in dependence upon the field strength of an external magnetic field. Typical sensors of this type are, for example, the sensors referred to as xe2x80x9cKMZ 41xe2x80x9d of Philips Semiconductors.
These sensors supply output signals which are sinusoidal as regards the angle of rotation and are electrically phase shifted by 90xc2x0 with respect to one another. The signals may be considered as real and imaginary parts of a complex number so that determination of the angle by means of the arc tg function is possible. The angle is determined by means of an algorithm known by the name of xe2x80x9cCORDICxe2x80x9d in angular measuring systems which are currently on the market and consist of, for example, a combination of sensors marketed as xe2x80x9cKMZ 41xe2x80x9d by Philips Semiconductors with an integrated electronic circuit comprising a signal-processing device also marketed by Philips Semiconductors as UZZ900x.
A sensor arrangement with a measuring error recognition feature is known from EP 0 927 868 A2, which comprises two sensors and one evaluation unit for determining a measuring signal from two sensor signals supplied by one sensor each. Such a sensor arrangement comprises, for example, two angle sensors and measures the angular position of a measuring object with respect to the sensor arrangement. To be able to determine errors occurring in such a sensor arrangement, particularly at the input of the evaluation unit during conversion of the sensor signals into A/D transducer units, the teaching of EP 0 927 868 A2 provides switching means for switching between two switching states in such a way that a sensor signal is applied in each switching state to the AID transducer units, the assignment of the sensor signals to the A/D transducer units in the two switching states is different, and the evaluation unit is formed for determining an error signal from two measuring signals found during each switching state.
DE 198 49 910 A1 discloses an offset-compensated angle measuring system. In the arrangement described in this document and used for measuring an angle between a magnetic field and a MR sensor, which supplies at least two 90xc2x0 phase-shifted electric sensor signals which are supplied to an A/D converter preceding an angle computing arrangement, the sensor signal value of the two sensor signals is determined in accordance with a predetermined formula from which the change of the sensor signal value is determined in dependence upon the computed angle and, in dependence upon this change of the value, an offset control of the sensor signals is performed, all this for automatic and continuous offset compensation of the static and dynamic offsets in an arrangement for computing the value.
German patent application 100 22 175.0 of Jun. 05, 2000 describes an arrangement for angular measurements, comprising an angle sensor supplying two mutually 90xc2x0 phase-shifted sensor signals whose amplitudes are dependent on the temperature. To provide the possibility of monitoring the flawless operation of the arrangement, the teaching of this application provides a temperature sensor and a microprocessor in the arrangement described in this document. In dependence upon a temperature value supplied by the temperature sensor, the microprocessor computes expected amplitude values of the sensor signals, compares these values with the actual amplitude values of the sensor signals, and generates an error signal when the deviation between the expected and the actual amplitude values exceeds a predetermined limit value.
In the arrangement described in German patent application 100 22 175.0, the amplitude values of the sensor signals vary due to the manufacture and type of the sensor element used, for example, the sensor types xe2x80x9cKMZ 41xe2x80x9d or xe2x80x9cKMZ 43xe2x80x9d marketed by Philips Semiconductors. Moreover, there is a significant temperature dependence on the amplitude values of the sensor signals, which is to be limited with the aid of the temperature sensor. For this reason, the range of tolerances defined by the predetermined limit value in accordance with the teaching of German patent application 100 22 175.0, i.e. the admissible deviation between the expected and actual amplitude values of the sensor signals, must be maintained within a wide range.
The arrangements described have the drawback that only serious error conditions, such as the loss of the sensor magnet or breakage of electric connections, are recognized by the diagnosis methods used by these arrangements. A supplementary evaluation of the measured sensor signals as regards their accuracy and reliability is not possible with the arrangements described hereinbefore.
It is an object of the invention to provide an arrangement for angular measurement with which monitoring and evaluation of the measured sensor signals as regards their accuracy and reliability is possible.
According to the invention, this object is solved by an arrangement for angle measurement, comprising an angle sensor for supplying two sensor signals which are 90xc2x0 phase shifted with respect to each other and have actual amplitude values, and a signal-processing device for computing and/or making available expected amplitude values of the sensor signals in dependence upon predetermined and/or measured values of selected circuit and/or operation parameters of the arrangement for angular measurement, computing and/or making available a plurality of graded limit values for a deviation between the expected amplitude values and the actual amplitude values of the sensor signals, by which graded limit values a plurality of ranges of values is fixed for said deviation, comparing the expected amplitude values with the actual amplitude values of the sensor signals, determining the deviation between the expected amplitude values and the actual amplitude values, and determining the range of values in which the determined deviation occurs, and generating an error signal comprising reliability class information about the range of values in which the determined deviation between the expected amplitude values and the actual amplitude values of the sensor signals occurs.
Due to the measures according to the invention, the measured sensor signals can be monitored and evaluated as regards their accuracy and reliability in a simple and reliable way. In addition to the measuring error recognition disclosed by the known documents, an arrangement for angular measurement with an efficient diagnosis function is provided. The output signal is not only examined for extreme error conditions but also for small deviations. The result of the computation of the angle from the measured sensor signals, performed in the signal-processing device, is evaluated. This evaluation is expressed by the error signal and may be utilized for a reliability test when further processing an angle signal assigned to the angle computed from the sensor signals.
A high solution of recognizable deviations between the expected amplitude values and the actual amplitude values can then be achieved. A further requirement with a view to angular measuring systems of the type described can be satisfied in this way. Requirements such as increasing reliability of operation and improving the self-monitoring function of the angular measuring system can thus be met in a simple way.
In an advantageous further embodiment of the arrangement according to the invention, an individually digitally encoded bit code in the error signal is assigned to the reliability class information in accordance with the range of values in which the deviation between the expected amplitude values and the actual amplitude values of the sensor signals occurs. The condition for a reliable, digital evaluation of the reliability class information is thereby achieved.
A further embodiment of the invention is characterized in that a CORDIC algorithm is used in the signal-processing device for computing an angle from the sensor signals, and the radius determination provided by this CORDIC algorithm anyway is utilized for determining the actual amplitude values of the sensor signals. This provides the possibility of a very simple and low-cost realization of the invention in existing angular measuring systems.
In accordance with a further embodiment of the invention, the signal-processing device is adapted to generate an angle signal assigned to the angle computed from the sensor signals, and the error signal is supplied together with the angle signal by the signal-processing device, so that a simple and directly common further processing and evaluation is possible.
The arrangement according to the invention preferably comprises a control apparatus receiving the angle signal and the error signal and being adapted to perform different functions in dependence upon the reliability class information comprised in the error signal. Particularly, different control program runs can be realized in dependence upon the contents of the reliability class information, taking into account the value of the deviation between the expected amplitude values and the actual amplitude values.
A temperature signal representing a measured temperature value, but also a signal representing a characteristic of the angle sensor may be preferably used as selected circuit and/or operation parameters of the arrangement for angular measurement. To this end, the arrangement according to the invention may particularly comprise a temperature sensor for measuring a temperature value and for making available the temperature signal representing the measured temperature value. This temperature sensor is preferably used for measuring the temperature of the angle sensor, but other temperature influences by other parts of the angular measuring system can be compensated in the same way. Alternatively, by providing characteristics of the angle sensors used, the signal-processing device may also be put into a position in which the limit values and hence the ranges of values determined thereby for the deviation found between the expected amplitude values and the actual amplitude values of the sensor signals can be exactly adapted to the respective angle sensors. Influences of type-dependent differences of the characteristics of the angle sensors used, or even individual spreads of the characteristics of the type of angle sensor used can be at least substantially completely eliminated in this way, so that the generated error signals represent an exact copy of errors actually occurring in the angular measuring system.
The signal-processing device and/or the control apparatus are advantageously adapted for a plausibility test of the sensor signals supplied by the angle sensor, which test is performed on the basis of the reliability class information. The reliability of the measured sensor signals can thereby be graded, which may play a role in their further evaluation.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.